The present invention relates to a process for the production of a carbon monoxide stream from a feed stream comprising carbon monoxide, hydrogen, carbon dioxide, methane, nitrogen and moisture wherein said process comprises contacting the feed stream at elevated pressure with a layered adsorbent bed comprising:
(a) a pretreatment layer consisting of a pretreatment adsorbent in order to adsorb moisture preferentially over methane, nitrogen and carbon dioxide; and PA1 (b) a main layer consisting of a main adsorbent comprising a cuprous compound in order to adsorb carbon monoxide preferentially over hydrogen, carbon dioxide, methane and nitrogen; PA1 (1) The ability to reversibly adsorb water. Otherwise, the main adsorbent comprising a cuprous compound will be adversely affected as follows. Firstly, water vapor is strongly adsorbed by inorganic-based adsorbents and thereby reduces the effective CO capacity of the adsorbent. Secondly, water vapor may oxidize the active CuCl (a preferred main adsorbent) to inactive Cu(OH)Cl. Finally, water may also solubilize chloride containing species on the adsorbent surface and cause corrosion problems. PA1 (2) Limited or no adsorption of other bulk gas components comprising carbon dioxide, methane and nitrogen. Otherwise, these components will end up being in the carbon monoxide product since they will be subsequently desorbed along with the carbon monoxide during the product recovery sequence. PA1 (3) Limited or no adsorption of the bulk gas component comprising hydrogen. Otherwise, hydrogen will end up being in the carbon monoxide product since it will be subsequently desorbed along with the carbon monoxide during the product recovery sequence. As noted above, Golden's 3A molecular sieve zeolite does not satisfy this criteria since hydrogen is small enough to be adsorbed by 3A molecular sieve zeolite. PA1 (4) Acid resistance. Otherwise, contact between the main adsorbent layer and the pretreatment adsorbent layer will result in the migration of various species present on the main adsorbent layer (such as CuCl.sub.2, NH.sub.4 Cl and HCl) to the pretreatment adsorbent layer which in turn will result in dissolution of an acid unstable pretreatment adsorbent. This is particularly true if water breakthrough to the main adsorbent occurs. While carbon-based adsorbents are acid stable, most inorganic desiccants like Golden's zeolites are not. Thus, unlike the present invention, Golden requires an acid resistant sacrificial adsorbent between the pretreatment adsorbent layer and the main adsorbent layer. PA1 (5) A capacity to adsorb carbon monoxide. Otherwise, the pretreatment adsorbent is essentially inactive for carbon monoxide production.
The prior art teaches such a process. In particular, U.S. Pat. No. 5,531,809 by Golden et al. (the specification of which is hereby incorporated by reference) teaches such a process wherein the pretreatment adsorbent comprises a 3A molecular sieve zeolite.
The present invention is an improvement to Golden whereby an adsorbent comprising cuprous chloride on a carbon containing support is used as the pretreatment adsorbent instead of Golden's 3A molecular sieve zeolite. The use of cuprous chloride on a carbon containing support is an improvement to Golden because it satisfies all five of the following five desirable criteria for a pretreatment adsorbent. Contrast this with Golden's 3A molecular sieve zeolite which only satisfies the first two criteria.
The utility of the present invention's pretreatment adsorbent is unexpected since carbon-based adsorbents are largely hydrophobic and thus would not typically be considered for a dehydration application. The present invention has surprisingly found however that cuprous chloride on a carbon containing support adsorbs enough moisture to prevent destruction of the main adsorbent while still providing all the other benefits discussed above.
The prior art does teach carbon-based adsorbents for separating carbon monoxide from mixed gases but only as stand alone adsorbents and not with the preferred surface area and bulk density characteristics of the present invention's carbon-based pretreatment adsorbent. In particular, U.S. Pat. No. 4,587,114 by Hirai teaches an adsorbent for separating carbon monoxide from mixed gases wherein the adsorbent is obtained by impregnation of an active carbon support with various copper salts. The adsorbent produced is claimed to be stable with respect to the water content in the feed gas owing to the hydrophobic nature of the carbon support and thus no special dehydrating pretreatment adsorbent is utilized. Because, however, Hirai's adsorbent contains significant microporosity owing to its high surface area of 1200 m.sup.2 /g, Hirai has high impurity levels of carbon dioxide and methane in the evacuated CO product. U.S. Pat. No. 5,126,310 by Kratz et al partially overcomes this problem with a lower surface area (600 m.sup.2 /g) carbon-based adsorbent. Kratz's adsorbent, however, has a low bulk density (24 lbs/ft.sup.3), and thus lacks the required mechanical strength for operating in a pressure swing adsorption system.